xlsx: rename repeated table headers without retrying handed-out suffixes (#3213)

table_columns restarted its suffix search at 2 for every duplicate
header, lowercasing and hashing every candidate already in use, so a
table of repeated headers was exported in quadratic time. The next
suffix to try is now remembered per lowercased base.

crates/xlsx/src/sheet_write/parts.rs

@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+
 use super::*;
 
 /// Valid table name: letters, digits, `_` and `.`, starting with a letter, `_` or `\`
@@ -21,16 +23,24 @@ pub(super) fn table_name(name: &str, used: &mut HashSet<String>, n: u32) -> Stri
 pub(super) fn table_columns(t: &Table) -> Vec<String> {
     let w = t.range.width() as usize;
     let mut used = HashSet::new();
+    // The next suffix to try per lowercased base, so repeated headers don't retry every suffix
+    // already handed out.
+    let mut next: HashMap<String, u32> = HashMap::new();
     (0..w)
         .map(|i| {
             let base = t.columns.get(i).map(|c| c.name.clone()).filter(|n| !n.trim().is_empty()).unwrap_or_else(|| format!("Column{}", i + 1));
-            let mut name = base.clone();
-            let mut k = 2;
-            while !used.insert(name.to_lowercase()) {
-                name = format!("{base}{k}");
-                k += 1;
+            let lower = base.to_lowercase();
+            if used.insert(lower.clone()) {
+                return base;
+            }
+            let k = next.entry(lower).or_insert(2);
+            loop {
+                let name = format!("{base}{k}");
+                *k += 1;
+                if used.insert(name.to_lowercase()) {
+                    return name;
+                }
             }
-            name
         })
         .collect()
 }
@@ -371,3 +381,38 @@ pub(super) fn vml_xml(sheet: &Sheet, n: u32) -> String {
     s.push_str("</xml>");
     s
 }
+
+#[cfg(test)]
+mod tests {
+    use gridcraft_model::TableColumn;
+
+    use super::*;
+
+    #[test]
+    fn repeated_headers_are_renamed_without_retrying_suffixes() {
+        let col = |n: &str| TableColumn { name: n.into(), totals: TotalsFn::None, totals_label: None, formula: None, unique_name: None, xml: None };
+        let mut columns: Vec<TableColumn> = (0..10_000).map(|_| col("Total")).collect();
+        columns[1] = col("total3");
+        let t = Table {
+            id: 1,
+            name: "T".into(),
+            range: RangeRef::parse("A1:NTP2").unwrap(),
+            header_row: true,
+            totals_row: false,
+            columns,
+            style: String::new(),
+            banded_rows: false,
+            banded_cols: false,
+            first_col: false,
+            last_col: false,
+            filter_button: false,
+            filter: vec![],
+            table_type: None,
+            connection_id: None,
+        };
+        let names = table_columns(&t);
+        assert_eq!(names.len(), 10_000);
+        assert_eq!(&names[..4], ["Total", "total3", "Total2", "Total4"]);
+        assert_eq!(names.iter().map(|n| n.to_lowercase()).collect::<HashSet<_>>().len(), 10_000);
+    }
+}